Prosthetic devices are utilized for replacing load-carrying skeletal members, such as the human hip, which are rendered non-functional due to acute arthritis, fracture, resections for malignancy or malformation. Such procedures have become more commonplace not only in human beings but also in animals such as dogs.
Hip joints are commonly repaired by total joint replacement with artificial components. Such hip prostheses typically include a femoral portion or component which is implanted in the femur and an acetabular component which is secured to the pelvis. The femoral component includes a head which articulates in a socket formed in the acetabular component.
With prostheses, especially a hip prosthesis, it is desirable to provide a rigid fixation of the prosthesis in order to provide long term stabilization, to minimize bone-implant micromotion, and to minimize the occurrence of complications, such as pain, after surgery.
Many known prosthetic devices require rigid fixation through the use of cement for the embedment of the prosthesis into the bone structure. These types of devices, however, display a number of disadvantages; during the installation of the prosthesis, it is typically necessary to wait, after sealing the shaft, until the cement has acquired sufficient resistance by polymerization before proceeding. During setting of the cement, the cement releases heat and can cause damage to surrounding tissue. The presence of the cement also inhibits the ingrowth of bone into the prothesis.
Another problem associated with prior art cemented prostheses is that they are designed to be firmly attached along their entire length. In the case of a hip prosthesis, the entire length of the stem of the femoral component is either cemented to the intramedullary canal of the femur to insure adequate stability. This causes compressive and other stresses created in and through the stem, when the leg is used, to be transferred to the femoral bone near the distal end of the prosthetic stem not uniformly along its length. The fixation created between the cement and the portion of the bone surrounding the lower portion of the stem transfers the forces developed through the ball joint to the lower portion of the femur and bypasses its proximal end. Over time, that portion of the femur is subject to deterioration of osteoporosis and thinning of the bone. As a result, the proximal end of the femur essentially loses density causing eventual loosening of the stem of the prosthesis within the bone. The problems associated with cemented implants have resulted in the development of implants which are inserted into the bone canals to obtain a press-fit arrangement.
In prostheses which are designed to be press-fitted, particularly in the case of press-fit hip femoral stems, two issues affect the clinical performance of the implant. These issues are initial implant stability and bone reactions. A stem design should focus on the tightest fit to resist subsidence and torsional forces and micromotion. The bone should be loaded most proximally in press-fit applications when good bone quality is encountered to stimulate appropriate bone remodeling, for continued implant support, and reduced stress-shielding of that critical region.
The present invention provides an improved geometry of the stem in the proximal bone region by a modification involving the shape of the implant and its interaction with the preparation instruments.
In prior art press-fit implants, an interference fit is provided by a slightly undersized preparation where the anterior and posterior faces of the proximal implant do not compress much bone relative to the medial and lateral portions. The anterior and posterior faces on many of the prior art implants are parallel sided providing no resistance to subsidence. Torsionally, the resistance can be minimal due to the lack of bone quality and contact. With a slightly tapered anterior and posterior face, the bone is slightly compressed in an interference fit. The angle defined by the tapered faces of typical prior art implants is approximately 3 degrees. Due to this slight angle, a linear distance corresponding to the implantation axis provides little outward displacement. Hence, the bone is not compressed to any great extent, which in turn, may contribute to less than optimal initial stability. In addition, the slight angle is not optimal in transferring compressive loads to the bone with most of the initial loads going to the medial proximal portions. Increased anterior/posterior loading occurs only when bone ingrowth occurs and bony remodeling occurs over a broad, mostly medial area. The present invention includes a geometry in the proximal region about the stem which provides a greater angle on the anterior and posterior faces of the stem. The angle on the anterior/posterior sides is at least double that of the prior art tapered stems and is located in a more proximal location.
The improved proximal locking zone of the present invention initially loads the bone more proximally and compresses the bone (in an interference fit) to a greater extent than prior art stems. This provides for densification of bone about the stem, greater resistance to subsidence, and greater resistance to torsional forces. In addition, the densification of bone potentially inhibits the transfer mechanism for implant debris to the boundary surrounding the implant/bone interface. A reduction in the incidence of lysis is also a potential benefit.
To aid in retaining the stem of the present invention, a bio-active material or the like, may be applied to the surfaces in the proximal locking zone. This material and such allow for bony ingrowth into the material which enhances the fixation of the prosthesis within the femur.
It is, therefore, an object of the present invention to provide a prosthesis which will transfer the forces generated in the upper portion of the stem to the proximal portion of the femur through an effective angle and thereby eliminate deterioration of the bone in that area.
It is a further object of the invention to provide a novel method of implanting a femoral stem which prepares the intramedullary canal for the novel geometry of the present invention.